During postnatal brain development, newly assembled neural circuits are refined through the strengthening of a subset of synaptic connections and the concurrent elimination of others. This process of synaptic refinement is first coordinated by intrinsically generated neural activity early in life and then driven by sensory experience during a later phase of postnatal development. Impairments in sensory-dependent refinement are thought to contribute to a heterogeneous array of neurodevelopmental disorders, consistent with the contribution of environmental risk factors, such as immune challenge, to their pathophysiology. Yet, therapeutic strategies aimed at correcting such impairments have been limited by a fundamental lack of insight into the molecular and cellular mechanisms through which sensory input refines developing circuits. The key focus of the proposed research is to obtain a more comprehensive understanding of the basic cellular and molecular mechanisms by which sensory experience refines synaptic connections in the course of development, and to determine how impairments in these mechanisms may contribute to neurodevelopmental disease. In preliminary studies, the applicant applied newly developed single-cell transcriptomic methods to identify Fn14 as a gene that is transcriptionally induced by visual experience in excitatory neurons and encodes a cytokine receptor that is required for sensory-dependent refinement in the developing brain. This proposal contains a comprehensive training and research plan to build upon these findings by addressing the remaining gaps in knowledge experimentally in the short-term, and by facilitating the establishment of an innovative and multidisciplinary research program focused on addressing these questions in the long-term. In Aim 1, the applicant will learn and apply specialized acute slice electrophysiology techniques to test the hypothesis that TWEAK, the pro-inflammatory cytokine ligand of Fn14, is necessary and sufficient to drive sensory-dependent refinement via a microglia-to-neuron signaling axis. In Aim 2, the applicant will combine electrophysiology with newly learned methods in bioinformatics to determine whether heightened TWEAK/Fn14 signaling contributes to excessive synaptic refinement in the maternal immune activation (MIA) mouse model of aberrant brain development. In Aim 3, the applicant will test the hypothesis that TWEAK/Fn14 signaling mediates functional refinement by promoting the conversion of immature synaptic spines to mature spines. These technical training activities will be augmented by focused mentorship from several highly successful scientists who are committed to aiding in the applicant's acquisition of professional and intellectual skills in the highly energetic and collaborative training environment at Harvard Medical School. These activities are expected to facilitate the successful transition of the applicant into an independent research position at an academic institution.